On a wooden gantry 30 metres above Glasgow’s busy city centre, specialist masons are preparing to remove the first layer of stonework from the west wall of the Mackintosh library.

After months of preparation and detailed research, work is now beginning in earnest on the restoration of the Mack library which, along with the rest of the west wing of Glasgow School of Art’s Mackintosh building, was engulfed by flames as students were preparing for their final-year degree show in May 2014.

About 90% of the grade A-listed building, which was completed by Charles Rennie Mackintosh in 1909 and is still considered his masterwork, was saved. But the library, one of the world’s finest examples of art nouveau design, which housed many rare and archival materials as well as original furniture and fittings, was almost entirely destroyed.

Within the library itself, a forest of steelwork is securing the external structure before two massive stone window piers are removed for assessment. The banks of debris and blackened wood have been cleared to make room for tidy scaffolding poles.

Joiners work in the professors’ studios at Glasgow School of Art. Photograph: Jeff J Mitchell/Getty Images

At the height of the fire, the building’s stonework endured temperatures of up to 1000C (1832F), and was then cooled down very quickly by the gallons of water used to douse the flames. This has left some of the stone shattered and fissured and and too weak for reuse.

Every stone is individual, made with skilled tooling and subtle carved profiles, and will be numbered when it comes out for testing

Since the summer of 2014, work in the library and throughout the Mackintosh building has been in constant progress, beginning with debris removal, then the painstaking salvage process conducted by specialist forensic architects, and the diligent drying out of the building to ensure against future difficulties with rot.

The restoration project’s main contractor, Keir Construction Scotland, expects involve up to 30 different trades and craftspeople, from horse hair plasterers to lead glaziers, many working with original materials that have been salvaged and preserved.

For example, forensic archeologists rescued 620 individual pieces of brass plate lamp fittings from the ash and debris in the library. The fragments were then sorted into “light kits”, dependent on their location in relation to the 48 original light fittings that had been hanging on the day of the fire, and enough useable material has been saved to remake 29 complete lights.

Restoration of the roof is also under way, using a replacement for the original southern yellow pine timber. This particular wood is not indigenous to Scotland but has been sourced from a mill in Massachusetts.

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The restoration team has also had to import tulip wood, the original timber used for the library’s internal fittings. Liz Davidson, senior project manager for Mackintosh Restoration, explains: “One of the major differences that people will notice on visiting the restored library will be the colour of the wood.

“It will be much lighter than it was in 2014 which had been darkened by over a century of use. The original timber was lightly stained to allow the grain of the wood to clearly show through – Mackintosh maintained this approach to the honesty of the materials he used throughout the building and it is an approach we are committed to honouring in the restoration.”

The GSA estimates that the cost of restoring the building could reach £35m. Alongside a simultaneous upgrade of the east wing of the Mackintosh building, the project is being supported by the £32m Mackintosh Campus Appeal, £18m of which has been raised to date including support from both the UK and Scottish governments. Trustees of the appeal include Brad Pitt and former GSA student Peter Capaldi.

Restoration of the Mackintosh building is expected to conclude by the end of 2018, with undergraduate students back in the building in 2019.